1. The study of calcium absorption and distribution in hypogonadal adolescent males has been extended to a group of six subjects. All subjects were studied at baseline and subsequently after a period of treatment that consisted of 2 injections each of l00mg testosterone enanthate spaced 3 weeks apart. Results show an average increase in calcium absorption of 61% coupled with a 108% increase in whole body calcium balance. These results were consistent with similar observations of increased protein synthesis rate determined from measurements of leucine metabolism as measured by glutamate oxidation and keto-isocaproic acid production from leucine as measured by our collaborators. 2. Short-term calcium kinetics measurements from 76 adolescent and adult women have been used to develop a clinical test for bone affinity for calcium. The test was developed by modeling short-term plasma tracer dilution kinetics using a power function clearance model. This model of calcium clearance gives a simple and reliable estimate of early bone accretion in the first three hours after injection of calcium isotope. The test is capable of differentiating accretion in adult from that in Tanner 1-3 adolescents at p < 0.001 using the Mann- Whitney Rank Sum Test. The model has been applied to the analysis of osteopenia in adolescent women with severe anorexia nervosa. The model has documented an increase in bone accretion of 158% after nutritional stabilization in a 16 year-old subject. 3. Studies of the crystallization kinetics of hydroxyapatite an other calcium phosphates have been used to develop a novel model of bone mineralization that depends on the Ca/PO4 ratio in salts. When the non-linear parameter of this equation is fit to crystallization kinetics or observed isotope dilution data from human studies we have found that: a) the fit in adult subjects is remarkably similar to that for the crystallization of hydroxyapatite b) the fit for data sets from children suggests the presence of Ca/PO4 ratios consistent with more soluble phosphates that might be expected to occur in the more rapidly turning over bone of immature subjects.